NE5565 electronic ballast controller - power circuit - circuit diagram - Huaqiang Electronic Network

The NE5565 is an electronic ballast controller developed by Philips. This bipolar monolithic IC integrates both a power factor correction (PFC) controller and a self-oscillating half-bridge driver, along with numerous advanced control and protection features. Utilizing the NE5565 in designing fluorescent electronic ballasts reduces the number of components needed, shrinks the overall size, cuts down on weight, and enhances the reliability and safety of the ballast.

1. Internal Structure of the NE5565 and Its Key Features

The NE5565 is housed in a 20-pin dual in-line narrow-body plastic package, as shown in Figure 1. The device primarily consists of a power factor (PF) amplifier, a DC error amplifier, a PWM controller, a half-bridge oscillator, an output buffer, a voltage regulator, a lamp voltage regulator, a lamp current rectifier, and undervoltage lockout protection, among other circuitry including overcurrent protection, as depicted in Figure 2. The pin functions and their respective input and output ratings for the NE5565 are detailed in Table 1.

The NE5565 incorporates two switching power supply control circuits: the first is a PFC boost converter controller that can enhance the electronic ballast's power factor to above 0.99, ensuring minimal current harmonic distortion while offering AC transient voltage protection; the second is a half-bridge oscillator circuit that converts the DC high voltage from the PFC output into a high-frequency AC voltage. The half-bridge controller drives two external high-voltage power MOSFETs to regulate lamp current, limit peak lamp voltage, and protect the power switch. The NE5565 operates within a temperature range of 0 to +85 °C.

The key features of the NE5565 include:

● Ability to perform both PFC and ballast dimming control on the same chip;

● Extremely low AC current harmonic distortion;

● Variable frequency mode;

● Programmable preheating and ignition for achieving a three-step soft start;

● Lamp overvoltage protection;

● Eliminates overshoot during load cutoff and implements overvoltage protection.

2. Primary Functions of the NE5565

The typical application circuit of the NE5565 is illustrated in Figure 3. Here, T1 represents a high-frequency transformer for the half-bridge oscillator, and T2 is a lamp current detection transformer.

2.1 Voltage Regulation (Voltage Regulator)

The 7.42V reference voltage output from the VREF pin serves as a reference for the control logic voltage. Vcc is typically 12.7V, and Vcc must be at least 9.3V before the VREF output is produced. The accuracy of VREF is ±3.5% within the temperature range of 0 to 85 °C.

2.2 Lamp Voltage Regulation

During preheating, ignition, and lamp shutdown conditions, the highest open-circuit voltage across the lamp load must be limited. In voltage regulation mode, the lamp voltage is controlled by the arc voltage of the lamp rather than the control circuit. When the VLAMP pin voltage exceeds VREF, the lamp voltage comparator detects the VLAMP pin voltage. During this period, the lamp voltage reaches the maximum allowable open-circuit voltage value, and the VLAMP voltage is reduced by the fast frequency increase circuit. The RXCX time constant determines the frequency offset time of the startup circuit (with a ratio of 2:1).

2.3 Low Voltage Lockout Protection

When the PFC and half-bridge control circuits should be turned on or off, the protection circuit uses the Schmitt trigger to detect the DC supply voltage of the Vcc pin and determines the upper and lower trip points of the supply voltage. The PFC and half-bridge control circuits remain off until Vcc rises from zero volts to the upper voltage limit (11V). Once Vcc exceeds the upper voltage, the PFC and half-bridge oscillator circuits begin to operate. When Vcc is below the lower limit voltage (10V), the PFC and half-bridge circuits are turned off. The PFC and half-bridge oscillators are not allowed to operate until Vcc exceeds the upper limit trip point. The minimum delay is set by the external component connected to the DMAX pin.

2.4 Lamp Ignition and PFC Overvoltage Protection

The half-bridge undervoltage lockout circuit samples the DC output voltage of the PFC. Before the PFC output voltage reaches the set value (such as 400VDC), the undervoltage lockout circuit prohibits the lamp from igniting. When the OV pin input voltage exceeds 5/7 VREF, the inverter frequency is normally ignited from the maximum value when the lamp is warmed up. At the lower frequency offset, the ignition procedure begins.

The overvoltage protection circuit prevents the PFC DC output voltage from exceeding the set value. When the overvoltage comparator input pin OV voltage is much higher than VREF, the PFC buffer gate drive output OUTP is turned off to prevent the PFC DC output voltage from further increasing. The overvoltage protection circuit only protects the overvoltage or overshoot generated by the PFC circuit and does not suppress the transient voltage of the AC line.

2.5 Capacitive Load Protection

A capacitive load protection circuit is used to prevent the half-bridge power transistor from failing when the lamp is removed. When the frequency exceeds the resonant frequency of the half-bridge LC load network, the primary voltage will lead the primary current. The protection logic detects the phase relationship between the LC network resonant current and the voltage. The IPRIM pin input voltage is the primary current signal of the LC network. If the IPRIM pin voltage is higher than -100mV (positive), that is, when the gate drive signal is high, the system malfunctions and the frequency of the half-bridge oscillator rises.

2.6 Half-Bridge Oscillator

The half-bridge oscillator is a triangular wave generator that generates a square wave signal to drive the buffer circuit. The oscillation frequency is determined by the resistance value and capacitance value of the RT and CT pins, and the CT pin voltage is the triangular wave voltage.

2.7 Output Buffer Driver

The output buffer is used as a level shifter to convert the low-level logic signals of the half-bridge oscillator and the pulse-width modulator into a 10V drive signal to drive the two power switches of the external half-bridge circuit. The OUTH half-bridge buffer/driver circuit drives an external level-shifting circuit and then drives the half-bridge power switch. The OUTP output can directly drive a MOSFET or a circuit that combines external level-shifting with a power MOSFET.

2.8 Pulse Width Modulator

The PWM control circuit is used to control the duty cycle of the PFC. The PWM frequency is determined by the half-bridge oscillator. The ramp voltage appears on the CP output pin and is synchronized with the half-bridge oscillator. Therefore, at the valley point of the CT triangle wave, the CP pin ramp voltage starts. When the CP pin ramp voltage exceeds the DC pin output voltage, the capacitor connected to the CP pin discharges. The resistors and capacitors connected to the DMAX pin control the maximum duty cycle, soft-start function, and half-bridge cut-off time.

2.9 Overcurrent Protection

The current value can be detected by a resistor connected to the CSI pin. When the CSI voltage is -500mV, the overcurrent protection circuit is triggered and the OUTP output is turned off. When an overcurrent occurs in the PFC input circuit, the capacitor on the DMAX pin is forced to discharge.

2.10 Power Factor Amplifier

To modulate the duty cycle of the PFC power switch, the rectified peak AC voltage and phase are sensed by the PF amplifier, and the power factor input is received through the PF pin. When the AC voltage reaches a peak and zero crossing, the input voltage of the PF pin is 1V and 0V, respectively.

2.11 DC Error Amplifier

This circuit is used to provide negative feedback of the PFC DC output voltage. The DC output voltage of the PFC is input to the DC pin through a resistor divider and filter network. The reference voltage of the DC error amplifier is VREF. The DC error amplifier output should be connected to a filter capacitor to eliminate switching noise.

2.12 Lamp Current Rectifier

A lamp current rectifier is used to provide negative feedback control of the average lamp current. The lamp current transformer (T2) and the load resistor convert the lamp current signal into a voltage that is applied to the L1 and L12 pins, and the CRECT pin provides a full-wave rectified output. External resistors and capacitors determine the gain and time constant of the circuit. The differential error amplifier compares the CRECT pin voltage to the internal 2/7 VREF reference voltage and adjusts the frequency of the half-bridge oscillator to minimize the error voltage to force the average lamp current to be constant.

In summary, the NE5565 offers a comprehensive solution for designing efficient and reliable electronic ballasts, providing multiple advantages such as reduced component count, improved performance, and enhanced safety.